This invention relates to a fluid-driven (motor) and/or fluid-driving (pump) machine having a plurality of working chambers of cyclically changing volume and valve means to control the connection of each chamber to low and high-pressure manifolds. The invention also relates to a method of operating the machine.
The invention has particular reference to non-compressible fluids, but its use with gases is not ruled out. It has particular reference to machines where the at least one working chamber comprises a cylinder in which a piston is arranged to reciprocate, but its use with at least one chamber delimited by a flexible diaphragm or a rotary piston is not ruled out.
With most fluid working machines the fluid chambers undergo cyclical variations in volume following a sinusoidal function. It is known to provide flow rectifying seating valves, allowing fluid to be admitted and exhausted from the working chamber, which valves are electro-magnetically actuated such that pumping and motoring strokes can be achieved. The chamber can be left to idle by holding the valve, between the working chamber and the low-pressure sump, in the open condition.
A shaft position sensor is used to provide the micro-controller with chamber phase information while flow or pressure demand inputs influence the rate at which chambers are pumped, motored or left idle. The micro-controller drives semiconductor switches, such as field effect transistors, which in turn actuate the valves connecting the chambers to either the high-pressure manifold or low-pressure sump.
Experience shows that varying the timing of the valves, such that portions of the stroke are disabled, in order to vary machine output creates a significant amount of audible and fluid borne noise.
The development of electro-magnetically actuated; seating valves working in conjunction with a varying fluid chamber volume, such as described in EP-A-361927 and EP-A-0494236, permitted the output of a fluid working machine having a plurality of working chambers to be varied, in a time averaged way, by the rate of selection of whole chambers as they became available at the ends of each expansion or contraction cycle.
EP-A-0361927 described the use of this technique for a pump in which shaft power was controllably converted to fluid power. EP-A-0494236 continued the concept and, by introducing a new mechanism for actuating the valves in a motoring cycle, developed the machine to allow a controllable bi-directional energy flow.
A multi-piston hydraulic machine according to EP-A-0494236 is shown in schematic section in FIG. 1. In the side wall of each cylinder 11 is a poppet valve 13 communicating with a high-pressure manifold 14 and in the end wall of each cylinder is a poppet valve 15 communicating with a low-pressure manifold 16. The poppet valves 13 and 15 are active electromagnetic valves controlled electrically by a microprocessor controller 20 feeding control signals, via optoisolators 21, to valve-driving semiconductors 22.
Pistons 12 act on a drive cam 23 fast to an output shaft 24, the position of the cam 23 being sensed by an encoder 25.
The controller 20 receives inputs from the encoder 25, a pressure transducer 26 (via an analogue to digital converter 27) and via a line 28 to which a desired output speed demand signal can be applied.
The poppet valves 13, 15 seal the respective cylinders 11 from the respective manifolds 14, 16 by engagement of an annular valve part with an annular valve seat, a solenoid being provided to magnetically move each said valve part relative to its seat by reacting with ferromagnetic material on the said poppet valve, each said poppet valve having a stem and an enlarged head, the annular valve part being provided on the head and the ferromagnetic material being provided on the stem.
In EP-A-361927 and EP-A-0494236, whole chambers were selected on the basis that valve actuation could be done during the instances of near zero flow. It was considered that delayed closure of valves, occurring during times of significant flow, such that part of the chamber displacement could be rejected, would result in extremely high rates of change of flow and pressure, which in turn would generate noise.
The approach of whole chamber selection works well for high flow rates, seeing as the mechanical payload, driven by this type of system, typically has a large momentum such that variations in flow energy cause relatively small changes in its velocity and, therefore, acceleration.
However, in practice it was found that whole chamber selection during times of low flow demand resulted in large flow variations, seeing as the fluid machine was idle for long instances between active chambers. When a payload has a small velocity, as it will when the actuating flow is low, the momentum will also be minimal. If each actuated chamber is considered to be delivering a quantum of energy to the payload, then the change in velocity will be significantly higher when the initial energy is low.